Add checksum to transaction page hash

[shesek]

Add a 4-byte checksum (four first bytes of sha256(hash-arguments)) to the hash, and verify it when the page is loaded.

It should probably also be advised for users to save the url, close the page and re-open it to ensure they have a correct copy prior to sending any funds to the multisig address (some browsers might limit the length of bookmarked urls, etc).